Minority Report

When one trait outperforms another, it is difficult to understand how diversity would persist with natural selection. One possible explanation is negative frequency-dependent selection, when rare traits have an advantage over common ones. Over time, different traits are maintained by evolution as their relative frequencies shift. This idea was recently experimentally probed by examining personality types in social spiders.

Jonathan N. Pruitt of McMaster University in Hamilton, Ontario, Canada, and four Canadian and American colleagues placed wild-caught African social spiders (Stegodyphus dumicola) in experimentally generated colonies and neighborhoods that varied by personality type. Individual spiders were designated as shy or bold based on lag time to resume activity after a simulated risky situation: a puff of air, mimicking an approaching predatory bat or bird, that causes spiders to pull their legs in to feign death. Shy spiders stayed tucked a long time, over 400 seconds. Bold spiders, in contrast, “pop[ped] right back out,” Pruitt says, in under 200 seconds – a risky move because the threat could still exist.

The researchers divided the assayed spiders into all-shy or all-bold colonies of ten individuals each. They then created spider neighborhoods by attaching five colonies to a tree, with six possible bold-to-shy colony ratios represented across different trees. After four months, the researchers recorded the reproductive success and survival of the colonies by monitoring the numbers of egg cases and living members present.

Bold colonies’ reproductive success was highest when surrounded by mainly shy colonies, producing over twice as many egg cases when they were the minority, compared to when in all-bold or 80 percent-bold neighborhoods. Shy colony reproductive success did not vary significantly with neighborhood personality, but shy colonies did outperform their bold neighbors in 80 percent-bold neighborhoods. Researchers suspect that, because bold colonies build larger webs and are more aggressive hunters, they bear a “costly societal engine” when competition from abundant bold colonies limits resources.

This important mechanism “basically allows for the maintenance of diversity through evolutionary time at the level of the group,” says Pruitt. If any strategy becomes too abundant, its performance declines, not just in individuals, but in groups as well. (Nature Ecology & Evolution)